The present invention is related to liquid cooling of a rotating X-ray target in an X-ray tube.
High powered X-ray devices of the type used in such fields as medical diagnostics and X-ray crystallography require an anode capable of dissipating a relatively large amount of heat. Since the primary mode of dissipating this heat is by radiative heat transfer from the anode, an increase in the radiating surface area, leads to greater heat dissipation. By rotating the anode, a fresh area of the target surface can be continuously presented to the beam of electrons emitted by the cathode and the heat generated during X-ray production can be advantageously spread over a larger area. Thus, anode rotation allows an X-ray device to be operated at generally higher power levels than a stationary anode device and the problem of target surface degradation found in devices that use a stationary anode is avoided, provided the temperature limits of the target surface material are not exceeded.
The amount of heat generated and the temperatures achieved by an X-ray device can be substantial. Since less than 0.5% of the energy of the electron beam is converted into X-rays, while a major portion of the remaining energy emerges as heat, the average temperature of the target surface of the rotatable anode can exceed 1200.degree. C. with peak hot spot temperatures being substantially higher. The reduction of these temperatures and dissipation of the heat is critical to any increase in power. The ability to dissipate the generated heat by anode rotation alone, however, is nonetheless limited. As a consequence, even though there has been a demand for ever higher-powered devices since rotatable anodes were first introduced, the development of such devices has lagged.
A further disadvantage of prior art devices is their limited lifetime, which is determined in part by their ability to dissipate heat. Since X-ray devices can be relatively expensive, extending the lifetime of such a device will result in substantial cost savings.
The time averaged heat dissipation of the X-ray tube used in a CT scanner determines the patient throughput. Present day CT scanner tubes dissipate approximately 3 kw. When the target of the X-ray tube overheats, as will happen if patient throughput is increased, the time between subsequent uses of the machine will have to be increased to allow the target to cool. An X-ray tube with higher heat dissipation will allow improved machine utilization.
When heated rotating discs need to be internally cooled to avoid temperatures that exceed design limits, direct liquid cooling can provide maximum heat removal. To maximize the heat transfer coefficients from the surface of the rotating anode, to the hollow interior of the anode very small passages carrying large coolant flows at high velocity is often not practical. Further, when it is desirable to use dielectric fluids whose heat removal capabilities are below that of water, the resulting heat transfer coefficients using conventional approaches are often too low.
It is an object of the present invention to provide a high intensity rotating X-ray target with high heat transfer coefficients over all internal surfaces to allow the use of a dielectric coolant.
It is another object of the present invention provide a high intensity X-ray tube target which does not require high coolant flow rates and complicated small coolant passage design.